Synchronize HTTP and D-Bus localization interfaces

[imobachgs]

Trello: https://trello.com/c/npg6apH0/351-make-l10n-http-json-and-d-bus-interfaces-share-the-same-state

At this point, two different (and disconnected) interfaces handle localization settings. The preferred one is HTTP-based, but a D-Bus one is still needed for internal communication.

This pull request tries reorganizing the code and keeping both APIs in sync.

Tasks

• Keep both APIs in sync
• Reorganize the code to decouple the D-Bus interface from the original Locale struct.
• Move common logic to the Locale struct (now L10n).
• Find a better name for the L10n struct.

[coveralls]

coverage: 71.819% (-0.3%) from 72.103%
when pulling 1d2a8db on sync-l10n
into 61853f4 on architecture_2024.

[lslezak]

The change looks reasonable, but I'm not a Rust expert... 😝

[imobachgs]

Thanks!